RIMFIRE CASE GAGE

An easy project for a beginning home shop machinist. Have some fun and possibly help with rimfire accuracy as they headspace relative to case rim thickness. Different rim thickness affects accuracy so with your new fixture you can group cases according to readings that are similar. A little testing will direct the grouping for you to use. Currently I am grouping with about .0002” per group. Time and target feedback will dictate how to proceed. With some factory ammo, I find up to .002 of an inch difference. Proper head space is important to be able to take advantage of select grouping.


I didn’t have the correct drill size necessary for the case diameter so I used a tool post grinder and cut down an old drill bit. I chamfered a small radius at the top to keep from indicating a case held incorrectly on a radius. I also use an indicator that is capable of reading in .0001 of an inch. Material used was 1 3/16 inch-12L14 which is easy to machine but you can use whatever you have available. Aluminum probably would work unless you run thousands of rounds through it and start to wear it out.

RIMFIRE CHAMBER IRON

Another tool for the toolbox is a chamber iron...........What is it?............ Have you seen or heard of not dry firing a rimfire firearm? If the firing pin projection is to great, upon pulling the trigger on an empty chamber, it may strike the edge of the chamber leaving a mark peening the metal into the chamber causing a new round not to fully chamber or a fired case to become stuck and possibly not eject.


Simply insert tapered end of rod into chamber and tap with a small hammer to “iron” out the peen the firing pin indented. Then use the slide hammer to un-stick the iron from the chamber. Repeat until the edge is back to original shape. Best to reshape a little at a time. There are other tools that have a flat ground to make the tapered end a “D” shape whereas you would insert it into the chamber and then turn it with the L shaped handle to remove the burr. These will work but there is not the same control as with the punch type. Trial and error will find the one you prefer.


This chamber iron is made from oil hardening drill rod, heated cherry red and quenched, then polished and again heated slowly until a medium to dark blue appears which indicates annealing back to about 52 to 55 Rockwell “C” scale. Tough but not extremely brittle. If left as treated, it would shatter upon severe use.